Experiments were conducted in a two-dimensional wind tunnel to study dynamics of large-scale structures in a planar compressible mixing layer. High-speed cinematic (300,000 frames/sec) Mie-scattering and shadowgraph techniques were employed to track the evolution of these structures in the mixing layer. A covariance algorithm was developed to obtain quantitative information about the size, shape, orientation and convection speed of these structures in the mixing layer from the digitally processed temporally resolved cinematographic images. The results for the pressure matched flows, show large ellipsoidal eddies in the mixing layer oriented at angles ranging from 25$\sp\circ-50\sp\circ$, which tilt away from and then towards the streamwise direction as they convect downstream. For the pressure mismatched cases, the eddies showed fluctuations in their angular orientation with time which scaled with the eddy passage frequency. The amplitude of these angular fluctuations was found to be a strong function of the non-dimensional streamwise pressure gradient parameter across the compression/expansion waves. The convection speed of the large-scale structures in pressure matched flows was found to be higher than that predicted by the isentropic relations, whereas for the pressure mismatched cases the convection speeds were found to be lower than the isentropic relation.Finally, the planar mixing layer formed by the Mach 1.67 stream bounded on one side, was passively excited using feedback acoustic waves. This was achieved without the aid of the helical and flapping instability modes present in other unbounded free jet flows. The present mixing augmentation was achieved by the formation of large vortical structures, which resulted in nearly a increase in shear layer thickness. This excitation technique involves judicious placement of acoustically reflective surfaces near (but not within) the shear layer. At least two acoustically reflective surfaces, one on the downstream side and the other acting as the lower wall are necessary for the excitation to occur. It was also confirmed that the excitation was caused by an upstream traveling wave initiated at the downstream surface. A plausible model was developed to explain the aero-acoustic excitation mechanism.